Inductively coupled plasma mass spectrometry (ICP-MS) is an analytical method that is capable of detecting metals and certain non-metals at concentrations at very low concentration, as low as one part in 1015 (part per quadrillion, ppq) on non-interfered low-background isotopes. The method involves ionizing the sample to be analyzed with an inductively coupled plasma and then using a mass spectrometer to separate and quantify the thus generated ions.
The plasma is generated by ionizing a gas, usually argon, in an electromagnetic coil, to generate a highly energized mixture of argon atoms, free electrons and argon ions.
Certain elements are known to have relatively poor detection limits by ICP-MS. These are predominantly those that suffer from spectral interferences generated by ions that are derived from the plasma gas, matrix components or the solvent used to solubilize samples. Examples include 40Ar16O for determination of 56Fe, 38ArH for determination of 39K, 40Ar for determination of 40Ca, 40Ar40Ar for determination of 80Se, 40Ar35Cl for determination of 75As, 49Ar12C for determination of 52Cr and 35Cl16O for determination of 51V.
One solution to this problem is provided by collision cell technology (ICP-CCT) that includes a collision/reaction cell that is positioned before the analyser. Into this cell, which typically comprises a multipole operating in a radiofrequency mode to focus the ions, a collision gas such as helium or hydrogen is introduced. The collision gas collides and reacts with the ions in the cell, to convert interfering ions to harmless non-interfering species or other ions that do not cause interference.
Due to the range of interfering species that may be present, in some cases it may be advantageous to use more than one collision gas. This usually means bleeding one type of gas into the collision cell, collect data thus obtained, and subsequently switch to another collision gas. The flow of collision gas is usually in the range of about 0.2 to 10 mL/min, and is typically controlled by a mass flow controller.
In principle, it would be advantageous to use a single mass flow controller to control the flow of different types of collision gas being used. However, due to the large dead volume of mass flow controllers, a gas flush of more than 10 minutes is required when switching over collision gases before data can be collected. Therefore, in current systems a separate mass flow controller is used for each collision gas being used. Since mass flow controllers are fairly expensive, this leads to significant added cost of each instrument.
It would be desirable to provide a gas control system that only requires a single flow controller but should ideally allow more rapid switching of gases in a simple, cost-effective manner.